This invention relates to an actuator for a motor-driven friction engagement device which does not utilize hydraulic pressure.
FIG. 13 shows a wet multiple disc clutch as an example of a conventional friction engagement device. In the wet multiple disc clutch shown, a clutch hub 102 is attached to the outer peripheral section of an input shaft 101 which is connected to driving means. A plurality of clutch discs 103 are provided around the clutch hub 102. A clutch drum 104 is rotatably supported around the input shaft 101. At one end of this clutch drum 104, a drive gear 105 is provided, and at the other, a plurality of clutch plates 106 are arranged between the clutch discs 103. A piston 108 is provided in the space defined by the clutch hub 102 and the clutch drum 104 through the intermediary of a return spring 107. The piston 108 is operated by supplying hydraulic pressure through an oil passage 109, which causes the clutch discs 103 and the clutch plates 106 to be friction-engaged with each other, thereby transmitting the torque of the input shaft 101 to the drive gear 105. By draining the hydraulic pressure of the oil passage 109, the piston 108 returns to the original position by virtue of the return spring 107, which causes the friction engagement of the clutch discs 103 and the clutch plates 106 to be released, so that the torque of the input shaft 101 ceases to be transmitted to the drive gear 105.
Generally speaking, the clamping force of this type of clutch, which is widely used as a control clutch, makes use of hydraulic pressure generated by its driving means. When controlling the clutch from outside, the hydraulic pressure is adjusted by means of various hydraulic pressure regulating valves, the desired hydraulic pressure control being effected by transmitting the right hydraulic pressure to the sections where it is required, through appropriate piping.
When a system is to be controlled from outside, an electrical signal is generally employed as the control signal. In the above-described conventional hydraulic actuator, however, oil is the hydraulic medium connecting the control signal, i.e., the electrical signal and the mechanical force for effecting the clutch engagement. As a result, problems arise from oil leakage through the piping, delay in response, impossibility of fine control, and a complicated mechanism, which lead to inadequate reliability.
In addition, the required hydraulic pressure is generally generated by driving an oil pump by means of a motor, which, however, is rarely used exclusively for the purpose of generating hydraulic pressure, so that some contrivance must be effected to make use of the hydraulic pressure. In some conventional actuators, an electric motor is exclusively used for the oil pump for generating hydraulic pressure. However, this causes a space problem. Moreover, it is not to be regarded as an efficient method, for it does not seem to be absolutely necessary to convert electrical energy, which is easy to control, into hydraulic energy.